i understand why you want to report him. it is not right for a professor to have a sexual relationship with their student, and i would find it concerning. but it kinda comes down to this: do you really want to go through the mess that will erupt from you reporting him when the facts are that you had a consensual relationship with him and there seems to be no coercion or exchanging of favors, nor was he teaching you. because once you report him, you cannot back out. you need to go through with it through the end. and believe me, other professors/staff will know and this will also affect your reputation (although it was not your fault). i am just wondering if you are aware of how much trouble it will also cause for you, and if you are committed to seeing it through.

because i was also in a similar situation, with a consensual relationship, and ultimately decided to not report for such reasons. either way i think you are in the right for reporting or not reporting him, because professors should not be sleeping with someone they know is a student at the same school.